1. Field of the Invention
The invention relates generally to radio antennas and more specifically to omni-directional antennas suited for use with global positioning system receivers.
2. Description of the Prior Art
Separate antennas for global positioning system (GPS) receivers are commonly provided for placement in locations that have clear visibility to orbiting overhead GPS satellites. Such antennas are then cabled to a GPS receiver inside a vehicle.
U.S. Pat. No. 5,173,715, issued Dec. 22, 1992, of which Eric B. Rodal is a co-inventor (Rodal, et al., '715), describes an antenna with curved dipole elements. Such an antenna comprises a base plate that forms a ground plane, a coaxial feed that also serves as a mast perpendicular to the groundplane and that supports the center of two orthogonal dipoles each formed of a pair of elements. The dipoles are implemented on opposite sides of a double-sided flexible printed circuit board.
The signals received by such antennas from orbiting satellites are at such exceedingly low levels that the impedance matching required from an antenna to a coaxial cable and from the coaxial cable to a receiver input, together with the signal losses in the coaxial cable itself, can cause the signal-to-noise ratio to become unacceptably low.
There also exists an intense competitive environment between manufacturers of GPS receiver systems. The manufacturing costs of all the components, the antenna and pre-amplifier included, can significantly influence the number of units that can be sold, because the manufacturing costs set a bottom threshold for pricing strategies.
The antenna described by Rodal, et al., '715 uses a double-sided printed circuit for its antenna elements and a rigid printed circuit board for a groundplane. Such components perform well, but are costly to produce. A less expensive structure to manufacture is needed that can simultaneously address the signal-to-noise ratio problems associated with GPS carrier signal reception.